Add new multipart template model additions added in 3.8.0

This PR adds the new multipart template model additions added in 3.8.0

Submitter checklist

[x] The PR request is well described and justified, including the body and the references
[x] The PR title represents the desired changelog entry
[x] If the change against WireMock 2 functionality (incompatible with WireMock 3), it is submitted against the 2.x branch
[x] The repository's code style is followed (see the contributing guide)